This invention relates to cooling apparatus for horizontal continuous casting of metals and alloys, particularly steels, and more specifically to aftercoolers for such systems.
In horizontal continuous casting systems, the molten metal coming out of the melt distributor is poured into a horizontal continuous casting ingot mold made of a heat-conductive metal and customarily cooled with a cooling medium. In the mold, upon profiling, the metal strand being formed begins to solidify and to consolidate starting from its outer surface and progressing inwardly. The solid strand shell formed during this process increases in thickness during its passage through the ingot mold. However, at the point of leaving the ingot mold, the shell is still relatively thin, in the case of metals and alloys customarily cast in such systems. The drawn off strand thus is not yet sufficiently mechanically sturdy for handling such as, for instance, for drawing off out of the ingot mold.
Consequently, downstream of the ingot mold in the direction of travel of the strand, there is generally arranged one or more aftercoolers in which a thickening or reinforcement of the strand shell is obtained, increasing the strength of the strand so that the strand, which in its center is still molten, can, without risk of breakage, be picked up by the strand withdrawal device, for instance by its driving rollers, and can thereafter be handled further as desired.
To stabilize the strength of the strand as rapidly as possible after its shaping, an important factor is to insure that the cooling within the ingot mold is uniform about the circumference of the strand. From experience gained in the continuous steel casting of billets and blooms, it is known to increase the heat dissipation by means of a universal conical design of the ingot mold cavity, tapered in the longitudinal direction of the strand, which promotes growth of the shell. It has also been described in the literature how to determine the degree of taper of the ingot mold with respect to the shrinkage of the metal in order to achieve the positive effects of improved heat dissipation and shell growth, accompanied at the same time by reduced ingot mold friction. Any initially optimally set ingot mold geometry undergoes changes during operation as a result of wear and/or warping, so that, for instance, the predetermined conicity is lost or there occurs possibly even a reverse conicity. Such an unfavorable ingot mold geometry may then result in damage to the cast strand, e.g., cracks or breaks.
It is also known that in determining the conicity of the ingot mold, the carbon percentage of the steel being cast and the differences arising therefrom with regard to dissipation of heat and ingot mold friction are to be taken into consideration.
In order to avoid damage as a result of wear and/or warping of the ingot mold it is customary in practice to check the ingot mold geometry by means of gauges during plant idle periods, which involves the performing of costly measurements.
In European Patent Application No. 26,487, there is described a process for the monitoring of the ingot mold status while casting operations are in progress, which makes it possible to recognize early any undesirable changes in ingot mold geometry and to prevent thereby the above described impairment of the strand, such as, e.g., cracks or breaks.
In that process, the actual value of the cooling capacity of the ingot mold is determined and compared with a theoretical value predetermined as a function of the carbon percentage and the residence time of the cast steel in the ingot mold. An excessive deviation of the actual value from this theoretical value indicates a potential damaging alteration of the ingot mold geometry. Appropriate measures are then taken to guarantee desired strand quality. While this known process makes it possible to recognize in advance the likelihood of damage to be suffered by the strand because of unfavorable ingot mold geometry, correction or readjustment of the ingot mold geometry during the operation is however not contemplated with that process.
European Patent Application No. 26,390 discloses a process for setting the rate of adjustment of the narrow sides of a plate-type ingot mold in the continuous casting of steel in which, for the purpose of changing the size, the spacing between the narrow sides of the mold is changed while the continuous casting operation is in progress. In order to keep the length of the transition piece of the strand, that is, the portion of the strand between the prior cast format and the format to be newly cast thereafter, and to keep material losses at a minimum, the rate of adjustment is as high as possible, which involves the risk of the occurrence of bulgings and breaks on the strand.
In that process, the amount of heat discharged during the adjustment of the cooling medium is measured at the narrow sides of the mold, and the spacing between the narrow sides is adjusted at a rate only fast enough that the amount of heat dissipated does not go below an amount predetermined in a given case.
Neither of the two last-mentioned prior art processes contemplates adjustment of the position of the sides of the ingot mold, other than its narrow sides.
German AS No. 2,415,224 discloses a process for the control of the cooling capacity, likewise only of the narrow side walls, of plate-type ingot molds during continuous casting, where the narrow side walls are clamped between the wide side walls and where, prior to the onset of casting, the mold cavity between the narrow side walls is provided with a taper converging in the direction of travel of the strand and adapted to the grade of steel and the width of the strand. Prior to the commencement of casting, the taper is adjusted additionally to a theoretical value corresponding to the predetermined casting rate and/or casting temperature, and, upon deviation of the casting rate and/or the casting temperature during casting operation, the taper is modified according to predetermined theoretical values corresponding to these changing casting parameters.
All of the known processes hitherto mentioned concern adaptation of the geometry of the casting mold to the dimensional changes in the strand that occur or that may be expected as a result of the changes in casting parameters, the grade of the metal or the alloy or, in the event of desired cross-sectional changes of the strand, where changes are made only in the position of the narrow side walls of plate-type ingot molds. These processes do not take into account the two other sides of the strand nor the dimensional changes occurring in the cast strand upon further cooling after it leaves the profiling casting mold, the resulting shrinkage phenomena, phase changes, and the like. However, these other factors have an important bearing on the strength of the finished strand and for the homogeneity and quality of the cast. It is precisely during further cooling after leaving the profiling casting mold, by means of an aftercooler or aftercoolers, that changes in the cross-section of the strand, generally reductions, occur, with the possibility of these changes in cross-section taking place nonuniformly as a result of the phase changes occurring at different temperatures. In other words, dimensional constancy can occur, for instance, in spite of cooling.
In addition to the ingot molds adapted to compensate for cross-sectional dimensional changes in the strand in one direction, aftercooling devices have become known in continuous vertical and arc casting plants in which cooling of the strand is effected by the application of a cooling medium, generally water, directly onto the strand. Austrian Pat. No. 303,987 discloses such a device where control of the amount of cooling water applied onto the strand is accomplished by means of sensors which determine the surface temperature of the strand prior to its entrance into and following its emergence from the aftercooling zone, and a central computer which processes the temperature data and controls the cooling water supply to achieve the desired cooling characteristics.
A similar device is disclosed in the German OS 1,932,884 which provides for control of different functions of a continuous arc casting plant. This device also effects control of the aftercooling device, operating likewise according to the direct cooling principle, on the amount of cooling water transmitted onto the strand. In the plant described in this German OS, control of the cooling capacity of the ingot mold, with the aid of temperature and thruput sensors that determine the amount of heat dissipated by the cooling medium, is also effected.
With the two last-described aftercooling devices, the dimensional changes in the strand do not cause any problems because of the direct application of the cooling medium onto the strand. However, direct contact between the cooling medium and the strand presents considerable drawbacks, such as evolvement of steam, nonuniform cooling and possibly reactions between the metal and the cooling medium.